Whenever I try to copy and paste a long command into a command block, it seems to work, but then when I click "done," and then go back into the command block, the command is erased. I don't know what I am doing wrong!
How exactly are you doing it? I'll show you how I do it.
Grab a command block, and place it down. Open it up, and do CTRL+V to paste it in the command block. Click DONE, make sure not to click close.
Are you on a multiplayer server? If you are make sure command blocks are ON because if they are not, it will not work.
Also do you have cheats on, or have OP on the server? If you don't, it wont work either.